A semiconductor diode laser is a monocrystalline pn junction device. In one form of such a device, the pn junction is in a plane disposed in an active region between two parallel rectangular faces of a monocrystalline semiconductor body. Two mutually parallel reflective faces that are perpendicular to the pn junction form a laser cavity. Lasing action is produced by applying a forward voltage across the pn junction. The forward bias injects electrons and holes across the pn junction. Electrons and holes recombine in the active region to cause stimulated emission of the radiation. Above a given level of electron injection, called the threshold current (I.sub.TH), emitted radiation is collected and amplified in the active region. The amplified radiation exits the active region parallel the pn junction as a monochromatic beam.
Electrons can be injected into the active region without stimulating emission therein. For example, they can escape outside the active region to adjacent portions of the semiconductor body, where they recombine without contributing to laser emission. Analogously, photons produced in the active region can escape from the active region by radiation in a direction not parallel the pn junction. In addition, it is possible for electrons to disappear within the active region without producing the desired emission of radiation, such as by combining with holes at crystal defects. All such losses reduce laser efficiency, i.e. output power. One can restrict escape of injected electrons and holes and stimulated photons from the active region by sandwiching the active region between two layers of monocrystalline semiconductive material having a larger energy band gap and a lower index of refraction than the active region. On the other hand, the active region, and as a practical matter the two adjacent layers must be a very high monocrystalline quality. This requires that these layers and the active region be closely matched not only in crystal structure but also in crystal lattice size. Moreover, one of the sandwiching layers must be doped to n-type conductivity and the other to p-type conductivity. Such a structure is referred to herein as a double heterojunction semiconductor diode laser.
Highly efficient double heterojunction long wavelength semiconductor lasers have heretofore been difficult to make. By long wavelength I mean longer than about 7 micrometers. I have found a material which permits one to make such a laser easily.